A cellulose acylate film is used as a base of a photographic film due to its toughness and flame retardancy. Moreover, as excellent in optical isotropy, the cellulose acylate film is also used as a protective film for a polarizing filter in a liquid crystal display (LCD) whose market becomes larger recently.
The cellulose acylate film is usually produced by a solution casting method. In the solution casting method, a dope containing cellulose acylate, additive and solvent is cast onto a running support to form a casting film. After having possessed a self-supporting property, the casting film is peeled as a wet film. The wet film is dried and wound as the cellulose acylate film in a roll form.
In the solution casting method, fine particles are generally added to the dope for the sake of scratch resistance of film, improvement of film conveyance, stick resistance of wound film, and the like. The fine particle is also referred to as matting agent, blocking agent, anti-creak agent, and the like, and has conventionally been used. When the fine particles are mixed in an entire film, it may deteriorate the optical properties of the film. Therefore, when the film is a multilayered film, the fine particles are preferably mixed only in an external layer of the film. As disclosed in Japanese Patent Laid-open Publication No. 2003-236863, a dope is separated into two: one is for forming an internal layer of a multilayered film and the other is for forming an external layer of the multilayered film, and matting agents are mixed only into the dope for the external layer. Moreover, dilute liquid is mixed into the dope for the external layer since the thickness of the external layer is preferably smaller than that of the internal layer.
The matting agents and the dilute liquid are mixed into the dope for the external layer according to the following conventional method shown in FIG. 7. According to this method, the number of inline mixers can be reduced. Dilute liquid 2A and matting liquid 2B that contains matting agents and cellulose acylate are firstly joined together. A mixture 2 of the dilute liquid 2A and the matting liquid 2B is added to a dope for forming an external layer of a multilayered film through an orifice 4 that is provided to a pipe 3. The dope for the external layer to which the dilute liquid 2A and the matting liquid 2B have been added is mixed by an inline mixer 6 provided with a number of mixing elements 5.
However, the conventional method has a problem in that a large amount of agglomeration of the matting agents is contained in the dope, which appears as extraneous matters in the produced film.